Thaw the frozen shrimp completely. Peel and devein them, then wash in lemon water and pat dry.
Grind the garlic, onions, coriander, and red chilli pepper with a pinch of salt in a mortar until you have a paste. Divide the paste in half. Toss the shrimp with one half of the paste and 1 tablespoon of olive oil, then refrigerate for at least 1 hour.
Heat enough olive oil to coat the bottom of a large saucepan over medium heat. Add the finely chopped tomatoes with the remaining half of the spice paste. Stir and cook for 2 minutes.
Pour in the coconut milk and stir well. Cover the saucepan and reduce heat to medium-low. Simmer for 20 minutes.
Add the shrimp with their marinade and the pepper rings. Cook uncovered over low heat for 5 minutes until the shrimp turn opaque throughout.
Season with salt and pepper to taste. Serve with basmati or Thai rice.